1. Field of the Invention
This invention is directed to a composite coating for maintaining low moisture levels at the surface of solid substrates. One embodiment of this composite coating comprises a layer of desiccant between two layers of the same or different elastomeric polymer. According to another embodiment of this invention, the coating comprises a layer of elastomeric polymer covered with a layer of an elastomeric polymer/desiccant mixture. The composite coating may be employed to encapsulate microelectronic integrated circuits (ICs) in order to inhibit moisture-related corrosion of the metal surface thereof.
2. Discussion of the Related Art
Micro-electronic circuits, such as thin film and hybrid integrated circuits, have found increasing use in sophisticated systems where it is necessary to assemble the circuits on printed wiring boards by various soldering techniques. The need for stable operating characteristics and increased operational life of such circuitry has necessitated an effort to provide the greatest possible protection of the circuitry from contaminants in the atmosphere. Hermetic sealing of the components is one way to provide a controlled environment in which the electronic device can function. Such protection generally involves vacuum packaging of the circuitry within ceramic or metal containers. However, a true and effective hermetic seal is difficult to achieve in all cases. Additionally, the hermetic container may contain water vapor and volatile components which, if not removed prior to closure, could affect the operation of the electronic component. In vacuum packaging of ICs, the IC surface does not contact the container and relies on a low moisture, vacuum environment surrounding the IC surface. This contrasts with plastic encapsulation, the other important method of isolating ICs from their surroundings. Plastic encapsulation comprises coating(s) applied over the IC surface directly.
Today a large proportion of ICs are protected from their surroundings via encapsulation in thermosetting or thermoplastic materials. Very often this is the only form in which some ICs are available. This situation has arisen because of the large demand for devices that comes from the consumer electronics industry which wishes to acquire its components at the lowest possible cost. Semiconductor devices encapsulated with resin compositions can advantageously be produced at low cost and on a large scale, as compared to those encapsulated with metals or ceramics. In contrast, however, resin encapsulants are generally inferior in humidity resistance and electrical properties, especially at high temperatures. For example, when an epoxy resin encapsulation-type semiconductor device is used at a high temperature and high humidity, electrical insulation of the semiconductor device may be lowered. This lowers the performance of the device by increasing leakage current or causing corrosion of aluminum electrodes or wiring, sometimes leading to the failure or breakdown of the wiring. One of the major failure mechanisms in thermoplastic and thermosetting material encapsulated devices is that of the corrosion of the microcircuit surface which is caused by the use of encapsulants which contain ionic impurities as additives and by-products from the manufacturer. The impurities can pick up moisture diffusing through the encapsulant and form an aggressive electrolytic solution. In the presence of a high bias voltage that is found between the conductors on the chip surface during operation, corrosion reactions occur which can rapidly cause device failure. Thus, as a result of the interaction between the moisture and the impurities existing within the encapsulating material, which is magnified at high temperatures, encapsulated ICs fail due to corrosion of aluminum metallization, wire bonds and the like.
Under certain very harsh operating conditions, e.g., in space or automotive applications, conventional thermoplastic or thermosetting material encapsulated devices are not as reliable as the more expensive hermetically sealed counterparts. This poses a particular problem for the automotive industry which requires devices that are capable of giving the utmost reliability while being of low cost. For example, it would be highly desirable if a low cost encapsulant could be found to protect the circuitry used in the harsh environment in the engine-compartment, which encapsulant would prevent moisture from reaching the circuit surface where it interacts with surface impurities to cause corrosion failure of the component.
A number of encapsulant coatings have been developed for ICs. For example, U.S. Pat. Nos. 4,079,511 and 4,511,705 teach the use of silicone resins as encapsulants for electronic circuits to avoid the penetration of moisture or gas. U.S. Pat. Nos. 3,264,248, 3,975,757, 4,327,369, 4,572,853, 4,614,963 and 4,617,584 are exemplary of patents which disclose the use of epoxy resins for encapsulating integrated circuits. In some of these patents, the epoxy encapsulant is taught to have special properties due to particular compounds incorporated therein. For example, the epoxy resin of '248 above is taught to be fire retardant due to the incorporation of phosphate compounds. In '853 above, the incorporation of organic phosphine compounds is taught to improve the moisture resistance of the epoxy resin. The addition of p-cresol to the resin of '963 above is taught therein to improve the corrosion inhibiting properties of the encapsulant resin. Still other patents, U.S. Pat. Nos. 4,163,072 and 4,542,260 teach a layered composite encapsulant of silicone/epoxy resins. In '072 above, the resin layers are cured together, the composite being described therein as thus having both moisture resistance and solvent resistance due to the silicone and epoxy, respectively.
As mentioned above, surface water and surface impurities do not operate independently. A threshold water content is involved, above which solid surface impurities dissolve to form aggressive electrolytic solutions. Corrosion rates increase many times at this threshold, which varies with the chemical form of the impurity. From the standpoint of low corrosion failure rates, it is clearly advantageous to operate devices with low-impurity and low-moisture content. Even in the vacuum packaging of ICs, attempts have been made to reduce surface moisture content. For example, Booe, in U.S. Pat. No. 4,081,397, teaches incorporation of a hygroscopic desiccant comprising alkaline earth oxides admixed with polymer which may be adhered to the inner wall of the container. This desiccant attempts to reduce the ICs' surface moisture without touching the IC surface directly.
The present invention provides a new means of achieving low humidity at a surface of a solid substrate by employing a novel composite coating thereon. While the usefulness of the present invention composite coating is directed primarily to microelectronic devices, the composite coating of this invention is useful in other systems subject to corrosion failure due to the presence of surface moisture, e.g., water pipes.